This application claims the priority of Japanese Application No. 2001-287994, filed in Japan on Sep. 21, 2001, the disclosure of which is expressly incorporated by reference herein.
This invention relates to continuous pickling and cold-rolling equipment and operating method thereof.
In recent years, a large number of continuous pickling and cold-rolling equipment systems, each of which has a pickling section in tandem to a cold rolling section, have been installed for improving a manufacturing process for high-grade steel plates typically represented by steel plates for automobile structure uses.
For instance, Hitachi Hyoron, page 80, Vol. 70, No. 6 (1988-6), describes an example of an entire configuration of one of such continuous processing equipment systems.
The continuous pickling and cold-rolling equipment shown in the Hitachi Hyoron is very large in scale and high in cost although the equipment offers excellent productivity and quality products yield. The number of mill stands in the cold rolling section such equipment includes is usually four to five.
High-grade steel plates, such as steel plates for automobile structure uses, are increasingly demanded to have 1) more formability and 2) higher strength than ever. A satisfactory response to this changing demand is becoming not probable as long as a continuous pickling and cold-rolling equipment in a conventional configuration is used.
This means that a cold rolling system and method should satisfy the following two requirements to simultaneously meet the above-mentioned two demands with high appreciation.
1. The rolling must give a large enough total reduction ratio. This means that the plate thickness before rolling (the thickness of the material, i.e., hot rolled steel strip) should be thicker as much as possible and the thickness of finished product thinner as much as practicable. Further, no intermediate annealing should be given in the cold rolling processes.
2. The rolling must be capable of rolling hard and strong materials.
These two requirements impose more severe conditions on cold rolling processes and cause the cold rolling equipment to be more powerful.
One of the conventional methods of effecting large-reduction cold rolling is a cold rolling method performed by increasing the number of stands on a rolling mill. For instance, there is an example in a tandem cold rolling mill that is equipped with six mill stands.
As stated before however, a tandem mill is a large-scaled and expensive facility by nature. Therefore, it would not be economical to increase expensive mill stands in such facility though responding to such new demands is inevitable. The amount of rolled products that should use hard and thick material usually occupies merely a small portion of entire rolled products, 20% or so at the most. Because of this, should the mill stands be increased to cover production of this 20-percent-products, the increased part of mill stands will not fully contribute to the production of the most of the rest of products.
Consequently, the conventional method, increasing the mill stands, would not be economical; the number of mill stands in the rolling facility should be within four to five as the conventional technique.
Another conventional method for effecting large-reduction cold rolling is such a method of reducing again, by cold re-rolling (DCR: Double Cold Reduction, sometimes referred to as DR), the material reduced once by cold rolling. For example, JP A 10-1284003 has disclosed this method. However, this technique cannot respond to the objective that the present invention deals for the reason detailed below. Because, in this technique, the material is usually cold rolled first then annealed before DCR-processing. The reason why the material is annealed before DCR-processing is the work hardening. The steel strip becomes hardened with cold rolling substantially causing too high rolling load to permit successive cold rolling in an ordinary manner. If further cold rolling is required over this work-hardened steel strip down to the desired thickness within a usual technique, annealing is necessary to reduce the hardness of the steel strip. Therefore, prior arts have necessarily applied annealing before DCR-process when obtaining a steel strip having certain thinness or thinner is desired.
However, the high-grade steel plates that will meet recent demands require a larger total draft in the cold rolling permitting no intermediate annealing in the cold rolling processes. This means that, if annealed, the retained effect, in a metallurgical sense, by cold rolling that will contribute to micronization of crystal grain would disappear. Accordingly, the effect of successive cold rolling in the DCR-process is limited to an extent that would be given by a draft usually available in a cold rolling after annealing. As a result of this, the micronization of crystal grain remains within an inadequate extent.
Therefore, the conventional DCR-process having an intermediate annealing cannot satisfactorily respond to recent demands. Thus, it is necessary to make it possible to effect cold rolling under such a severe rolling operation condition that a material is cold-rolled down to desired thicknesses without intermediate annealing in order to obtain finely micronized crystal grain.
In the conventional DCR-process, an annealed steel strip loses its elasticity and becomes easily bendable. This bendability in material caused by annealing makes handling of a steel strip very delicate and imposes difficulty on automating, tandemizing, and enlarging of equipment scale. Therefore, the conventional DCR-process is limited necessarily to a relatively small scale accompanied with special operating techniques: mostly a batch rolling method that rolls coil by coil of steel strips. This batch rolling method requires an expensive and complicated mill guide to pass a thin and easily bendable material to be rolled one by one into the rolling mills. Further, lower productivity and yield are inevitable because of batch handling.
It may be additionally mentioned that, in recent years, a continuous line has been installed which has an annealing section in tandem, or otherwise performs tempering rolling or cold re-rolling after the annealing. JP A 7-60305 is one example disclosing this style. These production lines permit a continuous operation by joining steel strips with each other at the entrance side of the line and perform tempering rolling and cold re-rolling. However, this style has been based on the condition that annealing should be given before application of tempering rolling or cold re-rolling. Therefore, this method cannot solve the objective matter that the present invention intends to achieve.
Further to the above, another method in a conventional cold re-rolling is described as follows. That is, there is a style in the DCR-process that repeats cold rolling without intermediate annealing. For example, when the number of mill stands in a production line is originally small or when cold re-rolling under different rolling specification is intended, the cold re-rolling has been applied without annealing even to common steels, and moreover applied to special steels particularly such as stainless steel or tool steel, and nonferrous metals. These examples all handle small amounts of production, and cold re-rolling therein are conducted in a batch style, even the style invites inefficiencies and low production yield. This processing style may be feasible for tandemizing. However, the tandem style is feasible only for rolling process, because a method for pickling or descaling for such special steels is different from that for common steels. Therefore, a tandem configuration of pickling with cold rolling is not feasibile, much less with inclusion of cold re-rolling. Thus, the conventional DCR-process cannot realize an economical and efficient cold re-rolling.